1. Field
Example embodiments relate to a method of fabricating a photo mask that may improve or optimize optical proximity correction (OPC).
2. Description of the Related Art
In general, a lithography technique used for fabrication of semiconductor devices involves transferring a pattern formed on a photo mask to a wafer through an optical lens. However, as the integration density of semiconductor devices increase, the size of mask patterns may approximate to the wavelength of a light source, resulting in the lithography technique being affected by diffraction and/or interference of light. Because an optical system for projecting an image may function as a low-pass filter, a photoresist pattern formed on a wafer may be distorted from the original shape of a mask pattern, as shown in FIGS. 1A and 1B.
When the size (or period) of the mask pattern is larger, the spatial frequency is lower. Thus, a light with various frequencies is transmitted through the mask pattern. As a result, an image similar to the original pattern is formed on the wafer. However, a portion of photo mask with a higher spatial frequency (e.g., an edge) may be distorted in a round shape. This distortion of an image is called an “optical proximity effect (OPE).” As the pattern size is reduced, the spatial frequency is increased such that the number of frequencies transmitted is reduced. Thus, a distortion of an image due to the OPE becomes worse.
An optical proximity correction (OPC) technique may be provided to overcome the OPE, which may be a principal cause of image distortion. According to an OPC technique, the shape of a mask pattern may be intentionally changed to correct the image distortion. OPC may lead to improvements in optical resolution and pattern transfer fidelity. OPC may use methods of adding/removing sub-resolution fine patterns to/from a mask pattern formed on a photo mask (e.g., line-end treatment or insertion of scattering bars). Line-end treatment may include adding a corner Serif pattern or a hammer pattern in order to overcome the rounding of an end portion of a line pattern as shown in FIG. 2A. The insertion of scattering bars may include adding sub-resolution scattering bars around a target pattern to reduce or minimize variation in pitches on the patterns with respect to pattern density as shown in FIG. 2B.
OPC has been considered a lithography technique, but currently is used as a design and CAD technique. A layout process may have been followed by design rule checks (DRC), electrical rule checks (ERC), electrical parameter extraction (EPE), and layout versus schematic (LVS) verification, but an operation of intentionally changing a layout using an OPC program may be added.
An OPC program may be categorized as either a rule-based method, processing layout data under rules prepared from lithography engineers' experience, or a model-based method, in which a layout may be modified based on the mathematical model of a lithography system.
Rule-based methods may comprise several rules including modifying a layout based on the rules and that a pattern may be partially cut or a small subsidiary pattern may be added and made beforehand. Rule-based OPC techniques may not correct the layout based on simulation results, a pattern formed on a wafer may not be as precise as required. But, rule-based methods may have an advantage of faster operating speed because layout data corresponding to the entire region of a chip may be processed at one time. However, it may be possible that many trial and error iterations are needed to apply a rule-based method to a new lithography process adopting different lithography apparatuses and/or a new illumination technique. Therefore, because of rapid technical developments, new rules based upon experimentation should be continuously made.
Model-based methods, adopting a mathematical model of an optical lithography system, may correct the deformation of a mask pattern by applying the model of the lithography system to a negative feedback system. Because model-based methods may be based on repeated calculation, a required operation time may be larger. Hence, the model-based method may be applied to a smaller amount of data. However, the model-based method may provide an improved or optimized OPC result, irrespective of the shapes of patterns. Further, model-based methods may find a solution even if a predetermined or given rule-set is not applied, and may be used to obtain a rule-set of a rule-based program. Thus, an improved or optimal solution is provided for various patterns with only a few experiments. As a result, when an improved or optimal solution is required irrespective of time, for example, in the case of a memory cell, a model-based OPC method may be preferred.
FIGS. 3 and 4 are process flow charts illustrating conventional methods of fabricating photo masks. Referring to FIG. 3, a conventional OPC method may include an OPC operation 30 of correcting a preliminary mask layout 25 using an OPC model 15. The OPC model 15 may be selected from a group of OPC models including various OPC models that are experimentally prepared based on the results of a lithography process using a test mask 10 having test patterns with various shapes and sizes. Considering the OPC model 15, it may be seen that the OPC operation 30 may be a model-based OPC process.
In operation 40, a photo mask may be fabricated using a mask layout 35 formed in the OPC operation 30. In operation 45, a process margin of the fabricated photo mask may be confirmed by analyzing the result of a lithography process using the fabricated photo mask.
When the fabricated photo mask satisfies a required process margin, the photo mask may be used for a lithography operation 50. When the fabricated photo mask does not have the required process margin, the photo mask may be rejected and a new photo mask may be fabricated. This re-fabrication of the photo mask causes an increase in fabrication cost and a delay in the date of delivery. In this case, the OPC model 15 may be inappropriate for the OPC operation 30 and a failure in the photo mask may be induced. Accordingly, it may be important to apply an appropriate OPC model in order to reduce the fabrication cost of photo masks and to improve productivity.
A conventional OPC method may select the OPC model 15 based upon the engineers' experience instead of providing a method for selecting an appropriate OPC model based upon the preliminary mask layout. As a result, the appropriateness of the OPC model 15 applied to the OPC operation 30 may depend greatly upon the engineers' experience. Further, the efficiency, or the operating time of the OPC operation 30 may be determined by the appropriateness of the OPC model 15. Therefore, when an unnecessarily strict OPC model is selected, the productivity of the OPC operation 30 may deteriorate. Because the appropriateness of the OPC model 15 depends upon the engineers' experience, this conventional OPC method may not reduce or prevent an unnecessarily strict OPC method from being selected.
As shown in FIG. 3, a conventional OPC method may further include a clip evaluation operation 20 of appreciating the appropriateness of the selected OPC model 15 by sampling a partial region of the preliminary mask layout 25. However, because the sampled region may be selected by engineers' experience, the dependence of the appropriateness of the OPC model 15 on the engineers' experience may not be completely overcome.
Referring to FIG. 4, another conventional OPC method may include an OPC virtual detection operation 60 of appreciating the appropriateness of the entire mask layout 35 using a virtual detection model before the photo mask is fabricated in operation 40. If it is determined that the evaluation result on weak point data 65 meets a predetermined or given standard in the OPC virtual detection operation 60, a photo mask may be fabricated using the mask layout 35 in operation 40. If not, the virtual detection model may be evaluated in operation 70. If the used virtual detection model is proper, the mask layout 35 may be denied, and a series of processes for forming a new mask layout may be repeated.
This second conventional OPC method may prevent or reduce an occurrence of a photo mask from being fabricated using an inappropriate mask layout 35, because the OPC virtual detection operation 60 may precede the operation 40 of fabricating the photo mask. However, the OPC model 15 may be selected by the engineers' experience, similar to the previously described conventional OPC method. Thus, this OPC method also may not overcome the dependence of the appropriateness of the OPC model on the engineers' experience. In other words, this conventional OPC method should also select an improved or optimized OPC model after trial and error. As a consequence, it may be difficult to improve the productivity of an OPC process.